Innumerable situations arise in equipment design that require controlled positioning and repositioning of a member (load). Numerous factors are involved in the selection of the type of actuator that is utilized to produce the positioning movements of the member, such as the nature of the equipment, the magnitudes of the forces required to both translate the member from position to position and to hold the member in a desired position, and the available power sources for the actuator. When fluid power, typically pressurized hydraulic fluid or compressed air, is available, a hydraulic (pneumatic) cylinder is the actuator of choice.
A conventional fluid cylinder includes a piston that is slidingly received in the cylinder to divide the cylinder into a pair of chambers, one to each side of the piston. The piston is typically mechanically linked to the member. If the fluid cylinder is of the single-acting type, pressurized fluid is selectively introduced into only one of the chambers by a fluid valve, while the other chamber is vented to a lower pressure (typically atmosphere), either continuously or selectively through the fluid valve. The piston is then propelled in the direction of the vented chamber, and the member linked thereto is driven in a desired direction from a quiescent position to an actuated position. Typically, the member is restored to its quiescent position by a return spring while both chambers are vented.
When bidirectional positioning of the member is desired, a double-acting fluid cylinder is utilized as the actuator. Here, typically separate three-way fluid valves are controlled to either vent the chambers or connect the chambers to the pressurized fluid source. When both chambers are connected to the pressurized fluid source, the fluid pressures in the chambers are balanced and the position of the piston is fixed to forcibly hold the member at a desired position. To move the member in either of two directions to a desired new position, the appropriate one of the chambers is vented until the member is driven to the new position and then is reconnected to the pressurized fluid source.
A particularly suitable fluid valve for controlling a fluid cylinder utilized as a positioning actuator is a three-way solenoid valve. The capability of electrically actuating a fluid valve to its several valve positions lends versatility and precision to controlling both positioning and repositioning of a member. Moreover, electrical control of fluid valve positions is readily conducive to microprocessor implementation.